Coal Flotation in a Centrifugal Machine

The aims of the project were:

• to develop a new type of flotation machine for the beneficiation of fine coal particles
• to test and demonstrate the machine by applying it to the flotation of coal in selected operating preparation plants
• to bring the machine to the point where it could be commercially applied

Method

The machine utilises a centrifugal principle, in which air is dispersed in a slurry in a rotating drum. Flotation occurs in a foam layer on the wall of the drum, and through the centrifugal effect it is possible to use very high air to slurry ratios, resulting in rapid flotation. A successful machine would have a very high unit capacity (two orders of magnitude higher) compared to any current flotation machine.

The centrifugal flotation device was first trialed at the Macquarie Coal Preparation Plant of Oceanic Coal Pty Ltd. Engineering problems resulted in a complete redesign and rebuild, followed by subsequent evaluation in pilot scale trials at Lemington Coal Preparation Plant in October 1995.

Two streams were tested, namely classifying cyclone feed (1.4mm x 0mm) and classifying cyclone overflow (minus 100mm). The effects of various operational parameters were investigated.

It was found when using the classifying cyclone feed as the feed stream, that high ash solids began to build up on the inner walls of the drum after about 20 minutes of continuous operation. This layer impeded and limited the air rate to the drum, resulting in the loss of all flotation performance.

The majority of the tests were completed using the classifying cyclone overflow as the feed stream to the drum. Although the sanding problems of the coarser feed were not apparent, performance was still poor. A low product ash (10 - 15%) was achieved although recovery and yield were poor (<50%).

It would appear that the major limitation to performance lies in the competing forces on the particles. It may be that the centrifugal forces on the larger particles entering the drum are too large for these particles to enter the inner froth layer and as a result will leave in the tailings stream.

Conclusion

Only the ultrafine fraction (-63mm), where centrifugal forces were minimal, did significant recoveries occur.

The operating range of the drum is therefore limited to -63mm material. Coarser feeds render the device inoperable.